The present invention relates to transducers whose active element is formed by a film of polymer material having piezoelectric and pyroelectric properties similar to those possessed by bodies belonging to certain classes of crystals. The acquisition of the properties in question is based on the creation of an electric anisotropy by means of an intense electric field applied perpendicularly to the principal faces of the polymer film. To obtain a high electric polarization, it is useful to stretch the polymer film beforehand, so that the macromolecular chains present a dipolar moment. The stretching of the polymer film precedes the electric polarization, so as to obtain a mechanical anisotropy by thinning down which is favorable to the formation of a polar phase.
The basic polymer material most currently used for producing piezoelectric or pyroelectric transducers is vinylidene polyfluoride (PVF.sub.2). Considering the small thickness of the film used for forming these transducers, it is advantageous to give it a self-supporting form by molding or heat-shaping along a non-developable surface. The form obtained is polarized electrically by means of electrodes covering these faces. These electrodes may be solid, liquid or gaseous, but the polarization operation requires in general the heat-shaped film to be replaced on a mold, for it tends to shrivel up as soon as it is heated to a relatively high temperature. This manufacturing technique is advantageous to the extent that it allows complex forms to be obtained which may correspond to very great extensions. However this method does not give entire satisfaction when the desired forms are slightly curved. The buttressing between fixed supports of a slightly curved form provides a movement amplifying effect which is particularly useful in practice to obtain a large vibratory amplitude. However, it is necessary to make sure that the forms using this effect have very precise dimensional characteristics. The technique of manufacturing by heat-shaping is then not well adapted to obtaining simple forms requiring great dimensional stability. It also has the disadvantage of requiring separate operations which are the shaping of the film and subsequently the electric polarization thereof.
In order to palliate these disadvantages, the invention provides shaping of the polymer film in the non-polar phase by stretching it over a metal former and by causing the relaxation of the mechanical tensions thus created under the action of the electric polarizing field transforming the non-polar phase into the polar phase.